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1. Field of Invention
This invention generally pertains to measuring the liquid level in a liquid holding tank, and, more specifically an accurate and dependable device to give continuous, empty to full readings.
2. Prior Art
U.S. Patent Documents found:
U.S. Pat. No. 5,747,689 issued May 1998 to Hampo, Momcilovich and Ross for fluid level sensing system.
U.S. Pat. No. 5,069,068 issued December 1991 to Jacob and Zawadzki for a fluid level maintenance sensor for use in a tank test system.
U.S. Pat. No. 3,935,741 issued February 1976 to Zinsmeyer, Johnson and Genz for level sensor.
Description of Related Art:
Liquid holding tanks are used for fuel, fresh water, sewage and other liquids on vehicles, recreational vehicles, boats, trains, buses, and aircrafts. Stationary holding tanks, above ground or underground hold various types of liquid. The level of liquid in all of these tanks must be constantly monitored to avoid running empty, running over or to determine needed service. Level-sensing devices currently have varying degrees of success.
Some level-sensing devices use floats that in turn operate rheostats, micro-switches, optical devices or magnetic switches. Others use electrical probes to detect a change in resistance as the amount of liquid changes in the tank. These devices require some type of electrical devices, probes, moving rods or other objects be placed in the liquid. Some liquids have a degrading effect on these devices causing malfunction over a period of time. Another device measures the temperature in a vertical arrangement on the outside of the tank. This method is only practical in cases where the liquid is a different temperature than the ambient air. Another method weighs the tank and compares the empty weight to the present weight of the tank. This method requires scales of some type or load cells if a reading at a remote location is needed. These devices are costly.
This invention uses the weighing of the liquid in the holding tank as a way to measure the level without weighing the tank. Instead, it weighs the liquid at or near the bottom of the tank using a differential pressure sensor. When the tank is empty, the pressure is zero compared to atmospheric pressure. As the tank is filled an accurate level measurement is made. In the water and sewage holding application one cubic inch of water weighs 0.036125 lbs. and produces the same pounds per square inch of pressure on the pressure sensor. Other liquids have varying weights and provide a corresponding amount of pressure. The pressure sensor, in turn, produces DC voltage corresponding to the pressure applied. The voltage is amplified and sent to a monitoring device such as a meter or other display device. A calibrated meter scale or display device indicates units and/or the percent of full tank capacity. This information allows the operator to plan for service.